TOYOTA 2010 Prius - Manual (Page 314)

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3-1. Using the air conditioning system and defogger
■
The rear window and outside rear view mirror defoggers can be oper-
ated when
The “POWER” switch must be in the ON mode.
CAUTION
■
When the outside rear view mirror defoggers are on (vehicles with mir-
ror defoggers)
Do not touch the rear view mirror surfaces, as they can become very hot and
burn you.
NOTICE
■
To prevent 12 volt battery discharge
Turn the defoggers off when the hybrid system is not operating.